This invention relates to speed control unit for controlling the speed of a pneumatic rotation motor. In particular, the invention concerns a speed control unit that includes a speed governor and an overspeed safety device for a pneumatic rotation motor having a stator with an air inlet passage and a rotor journalled in the stator.
One problem inherent in prior art speed control units including speed governors, for instance of the type described in U.S. Pat. No. 2,485,514, is that they are located at one end of the motor and, therefore, they add to the length of the motor. This previously known type of speed governor is also exposed to particles of all kinds entering the machine housing by the supplied pressure air. This means that this type of speed governor easily gets contaminated by foreign material and loses its ability to operate as intended.
Another prior art example of the above type of speed control unit is described in U.S. Pat. No. 3,708,240. This known speed governor is located in the rotor shaft and does not really add to the length of the motor. However, this built-in speed governor is still exposed to the risk of contamination, because the air flow to the motor passes right through the speed governor mechanism. Accordingly, foreign particles transported by the air flow passing through the governor may contaminate and cause malfunction of the governor.
In still another prior art device described in U.S. Pat. No. 3,071,115, there are provided both a flyweight operated speed governor and an overspeed safety device, both disposed within the rotor. Also the overspeed safety device is operated by flyweights, and the rotor design as a whole is rather complicated. Due to its location inside the rotor, these mechanisms are protected from dust and other particles transported by the pressure air. However, there are several guide surfaces for the flyweights and the valve element lock pins which after some time of tool operation may be exposed to corrosion or other types of sticking effect. This might jeopardise the intended safety function. Moreover, the flyweight type safety device actuator is not only complicated but requires a relatively large radial space. Still another drawback with this known type of device is the difficulty to make it operate with very fast rotating rotors, such as turbine wheels. In such applications, the centrifugal forces then acting on the flyweights and other parts are very strong, which puts high demands on for instance the dimensions and material of the flyweight springs.